Improve structure of package summary pages

Package summary pages currently do not have any links to its content sections in the sub-navigation bar. We should display links to the various summary tables (interfaces, classes, enums etc.) like we do on other types of pages.

Also, with the introduction of record classes, there are now seven summary tables for the various kinds of classes and interfaces. This makes things hard to find and requires a separate link for each summary table. It would help to merge all class and interface summaries in a single table, with a tab for each kind of class or interface.